Burner cap assembly

ABSTRACT

A combination of a steel burner cap, a burner cap mounting plate and a mixing tube. One end of the tube is adapted for connection to a gas manifold and the other end is connected to the mounting plate. Unobstructed openings intermediate the ends of the tube allow air to be aspirated when gas is injected from the manifold. The cap may be crimped to the plate for assembly tightness. Gas and primary air pass from the tube into the cap for further mixing while flowing radially out within the cap and subsequently through a plurality of ports. Ledges on the cap both above and below the ports extend radially outwardly to provide a flame shielding annular recess at the outer extent of the ports.

llnited States Patent [72] Inventor Albert F. Craver Bay Village, Ohio[21] Appl. No. 36,907 [22] Filed May 13, 1970 [45] Patented Dec. 7, 1971[73] Assignee The W. J. Schoenberger Co.

Cleveland. 01110 [54] BURNER CAP ASSEMBLY 9 Claims, 6 Drawing Figs.

[52] U.S. Cl 126/39 11, 126/214 R [51] Int. Cl F24c 3/00 [50] Field ofSearch 126/38, 39, 39 H, 39 N, 39 K, 44, 214, 40, 50; 239/559, 561, 567

[56] References Cited UNITED STATES PATENTS 1,196,230 8/1916 Graves126/39 UX 2,642,934 6/1953 Gunther et a1 126/40X 2,781,779 2/1957 Kindl26/39N UX 3,236,461 2/1966 Kindl et a1. 239/559 Shnidman, Gaseous Fuels,I954, pgs. 133- 154.

Primary Exam/new"hurles J. M yhre Ar!urney-Fay, Sharpe and MulhollandABSTRACT: A combination of a steel burner cap, a burner cap mountingplate and a mixing tube. One end of the tube is adapted for connectionto a gas manifold and the other end is connected to the mounting plate.Unobstructed openings intermediate the ends of the tube allow air to beaspirated when gas is injected from the manifold. The cap may be crimpedto the plate for assembly tightness. Gas and primary air pass from thetube into the cap for further mixing while flowing radially out withinthe cap and subsequently through a plurality of ports. Ledges on the capboth above and below the ports extend radially outwardly to provide aflame shielding annular recess at the outer extent of the ports.

PATENTEU DEC 71971 $1625; 196

sum 2 or 2 52 FIG.5

FIG. 6 (PRIOR; ART) 'INVENTOR.

ALBERT F. CRAVER M S/xaape MM ATTORNEYS BURNER CAP ASSEMBLY BACKGROUNDOF THE INVENTION The inventor intended to develop a new gas burner forthe gas range industry which would be economical in cost, have theproper configuration for the aesthetics of the housewife and overcomecertain undesirable operational features of conventional gas burners.

Gas burner construction has not changed substantially in years or so.Basically each burner includes (a) a gas supply unit, (b) a primary airinlet through a mixing tube connected to the gas supply, (c) an airshutter for regulating the size of the air inlet, (d) a venturi sectionin the mixing tube, (e) a burner base attached to the end of the mixingtube and (f) a burner head having ports about its periphery beingmounted on the burner base.

Voluminous American publications have described techniques and setstandards for gas burners over the past 50 years. Thus, much empiricaldata and technical formulas are available and recommended by varioustechnical groups. Substantially all such data are directed to slightmodifications of the above-listed basic elements.

Notwithstanding all of the previously recommended design techniques, theinventor decided to try something new. Because it was his intent tominimize cost he decided to eliminate the most expensive portion of theconventional gas burner which is the burner base. Conventional burnerbases are cast iron or die-cast aluminum alloy or steel drawn cupstructures of relatively large mass which serve as both a heat sink anda mixing chamber for the gas and primary air. The inventor eliminatedthe burner base altogether and substituted a tube of substantiallyuniform cross-sectional area from the gas manifold to the burner head.

Surprisingly, although the flames emanating from the resulting burnerhead were not perfect in many respects, they did give promising resultswhich warranted further investigation. The main drawback to the initialtrial was the high velocity through the burner ports which resulted insubstantial blowoff", excessive noise from improper ignition andinadequate combustion.

The variable parameters of a burner assembly for a conventional gasrange are relatively well settled. The industry over the years hasobtained many designs which have proved to be successful in meeting thestandards set by the gas industry and government regulations. Because ofthe many parameters involved, modification of one will tend to changethe results obtained. Customarily, what burner designers do is modifyone portion of a burner assembly to obtain a particular result and thenmanipulate the other variables to obtain combustion at an acceptablestandard.

Thus, it is not surprising that'the elimination of one of the parameters(namely, the burner base shown in FIG. 6) gave results which wereimmediately unsatisfactory by current standards. What was surprising andunexpected was that the results obtained gave promise that furthermodifications might be made to obtain acceptable burningcharacteristics.

Elimination of the burner base increased the internal pressure at theports because the gas-air mixture was delivered directly to the burnercap without any intermediate obstruction or any substantial pressuredrop.

With a conventional burner base the gas-air mixture from the mixing tubeenters a mixing chamber which results in a relatively large pressuredrop. Then from the mixing chamber it is distributed to the burner cap.The burner base conventionally has a volume of about six times that ofthe burner cap and the elimination of this volume from the burnerassembly is a great saving in materials.

Because of the elimination of the burner base and the reduction of thedownstream back pressure, the combustible gas injected from the gasmanifold tends to aspirate more primary air through the air holesintermediate the ends of the mixing tube. The increased primary air inthe gas-air mixture tends to increase flame ignition and cause furtherinstability in the flames.

After modifying several of the variable parameters and innovating acombination of structural elements, the inventor was able to obtain asatisfactory flame with acceptable combustion and operatingcharacteristics. This was accomplished by 1) eliminating the venturisection of the mixing tube, (2) designing a new cap with a new portconfiguration and (3) in some cases eliminating the air shutter at theprimary air inlet to the mixing tube.

First, the elimination of the venturi section in the mixing tube wascontrary to all technical recommendations and the known art of presentday burner construction. The venturi section formed in accordance withmathematical formula has always been thought necessary in order toaspirate enough primary air for the initial gas-air mixture. Theelimination of the burner base caused greater gas velocity past the airopenings and greater air aspiration. In turn, elimination o the venturisection reduced the amount of air aspirated through the air openings.Thus, as acceptable balance was maintained, notwithstanding that theprior art advises otherwise.

Second, it was conceived that what was needed in the burner portconfiguration to minimize blowoff was something to coalesce the flamesfrom the ports. The inventor concluded that this might be accomplishedby a cooperative port configuration consisting of large and small portswhere the small or secondary ports were located in such a position thatthe flames from small or secondary ports coalesced directly with theflames from larger or primary ports. In the final analysis and design,the result was a plurality of secondary ports to stabilize the ring offlame around the periphery of the burner cap and other secondary portsto carry over" the flame from one primary port to another.

Third, prior burner designs increased gas velocity by a venturi sectionin the mixing tube for the purpose of aspirating larger amounts ofprimary air and then reduced the velocity with the voluminous mixingchamber in the burner base. Eliminating the burner base eliminated theneed for the venturi section. Hence, with the elimination of the burnerbase and the venturi section the inventor was able to standardize thesize of the openings in the mixing tube and eliminate the conventionalair shutter BRIEF DESCRIPTION OF THE INVENTION A thin-walled tube havingopenings intermediate its ends for the aspiration of primary air isconnected at one end to a manifold. Combustible gas fed to the manifoldcould be any conventional heating or cooking gas commercially supplied,such as natural gas or liquified petroleum gas as is sometimes used incooking appliances in mobile homes and campers. The other end of thetube is connected to a burner cap mounting plate and a flat toppedburner cap is affixed to the mounting plate.

The whole assembly is supported by a bracket connected to the mixingtube and with its distal end resting on the upper surface of the oven ofa gas range or the bottom of the burner box.

Ports are provided in the burner cap which are directed substantiallyradially outward of the cap and ledges are provided both above and belowthe ports to provide a flame shielding recess at the outer terminus ofthe ports. The bottom ledge serves generally to prevent turbulence inthe port area which would cause blowoff and lifting. The upper ledgetends to retain the ring of flame at the low turndown" or minimum outputof the burner.

Objects of the invention will be obvious from a detailed reading of thedescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a prospective view of a gasrange FIG. 2 is an enlarged fragmentary plan view of a single burnerfrom FIG. 1 and incorporating the novel burner cap assembly of thisinvention.

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2.

FIG. 4 is a prospective view of the burner cap assembly of thisinvention.

FIG. Sis a sectional view taken along line 55 of FIG. 4. FIG. 6 is asectional view of a conventional burner as found in the prior art.

DESCRIPTION-OF THE PREFERRED EMBODIMENT FIG. 6 is illustrative of theprior art as relates to burners. Conventionally the burner cap assembly10 includes a mixing tube 12 leading from a manifold (not shown) to aburner base 14.

The mixing tube 12 would, of course, include a venturi section (notshown) somewhat along the lines shown in the patents to Vixler, US. Pat.No. 2,638,976, Blanzy, US. Pat. No. 3,l3 1,752 or Fulmer, U.S. Pat. No.3,219,098.

A holding plate 16 may be welded to the venturi tube 12 and attached tothe burner base 14 by screws 18. Conventionally, a gasket 20 must beused to prevent gas leakage at the joint. Three to six assembly screwsor bolts are conventionally used in the assembly.

The gaseous mixture from the mixing tube 12 is fed directly into a largemixing chamber 22 and subsequently into an annular channel 24 whichserves to feed and equally distribute the gaseous mixture to a burnercap 26.

The type of conventional burner construction and assembly illustrated inFIG. 6 has been thought necessary so that the burner itself will besufficiently flexible to satisfactorily burn all commercial gases understandard variable heating values and distribution pressures, and at thesame time meet the exacting standards for such burners as required bymost state and local ordinances and such national perfonnancerequirements as may be demanded by the American Standards Associationand Underwriters Laboratories.

The large internal gas volume of the conventional burner was considerednecessary to give an even distribution and equal pressure to the gas atthe burner cap ports 28, to obtain satisfactory flame stability and givegood flame definition under conditions of standard commercialdistribution. Commercial gas may vary greatly in B.t.u. content,specific gravity and pressure. The burner assembly may be required toperform with commercial gases where the required heating input capacitymay vary from approximately 500 to 12,000 B.t.u.s per hour.

As is evident, the large volume of the conventional burner base andburner cap in combination leaves a substantial volume of unburned gaswhen the supply is turned off. It is ax iomatic to say that the greaterthe volume of the unburned gas, the greater are the hazards from socalled flashback", noise of flame extinction and odors of unburned gas.

The odors of unburned gas are not really hazards, in fact. But they aretruly hazards to commercial sales. As is well known in the art, the odorone perceives with unburned commercially available gas is not, in fact,the odor of the gas itself. Natural gas is generally odorless. Theodoriferous additive is incorporated in the natural gas to serve as asafety feature for gas ranges for housewives. However, the resultingodor from the added ingredient in natural gas is a disquieting factorfrom the psychological standpoint in the home and it is extremelyimportant that such odors be minimized. Thus, there is a real need tominimize the volume of unburned gas.

The burner assembly of this invention minimizes some of the priorhazards as well as the volume of the burner cap assembly. FIG. 1illustrates a conventional gas range 30 having a burner box 31 and anoven 32 located below a plurality of gas burning units 34 on the top ofthe range. The top ofa conventional range usually includes an enameled,stamped sheet metal top 36 having apertures 38 for the burner assembliesas shown in FIG. 3. A pan 40 having an opening 42 in its center sits ineach aperture 38. A grate 44 is mounted over each aperture 38 above thepan 42 and a flat-bottomed utensil 46 (shown in phantom in FIG. 3) issupported by the grate 44.

The burner cap assembly 48, best seen in FIGS. 4 and 5, includes amixing tube 50 supported by a bracket 52, a burner cap mounting plate 54fixed to one end of the tube and a burner cap 56 crimped to the plate.It is not absolutely necessary that the cap be crimped to the plate butit is preferred where the range is in a mobile home or camper.

The mixing tube 50 is conventionally connected to a manifold andmetering device (not shown) which feeds combustible gas into the tube.Openings 58 allow the aspiration of primary air as the combustible gasis injected into the tube past the openings.

As seen in FIG. 3, the bracket 52 may be bolted at 60 to the uppersurface 62 of the oven 32. While the top of the over is shown as asingle metal plate it is obvious that this is merely illustrativebecause any conventional oven includes insulation, etc.

The connection between the mixing tube 50 and the burner cap mountingplate 54 comprises a shoulder 64 crimped into the mixing tube and anupper flanged section 66 which is staked down on the plate 54 after itis assembled on the shoulder 64.

A drop-on-type burner cap 56 slips down over the periphery of themounting plate 54 and may be crimped or staked at 68. A continuous flattop 70 on the cap 56 bridges the space across the annular vertical wall72.

A mixing chamber 74 is formed by the cap and plate 54 which serves tofurther mix the combustible gas and primary air delivered by the mixingtube 50. The gaseous mixture will flow through the tube 50 upinto thechamber 74 and will be caused to change its flow direction by the flattop 70. The result will be a rolling, boiling, mixing type of motioncreating turbulence within the chamber 74 as the gas-air mixtureprogresses outwardly toward the ports in the vertical wall 72.

The particular port pattern shown in FIGS. 3, 4 and 5 is effective toprevent flame blowing at the relatively high port velocity which resultsfrom the direct feed by the tube 50 into the chamber 74. Larger ports 76are the primary exit means for the gas-air mixture. They are preferablyround, approximately 0.078 inches in diameter and alternately spacedaround the cap 56 with a spacing of approximately 0.3 I25 inches betweencenters. Smaller ports 78 are interspaced between the larger ports 76and are of approximately 0.055 inches in diameter. Ports 78 have beenadded to give consistency of flame carry over between the primary ports76, particularly at low gas inputs.

Other secondary ports 80 of approximately the same size as secondaryports 78 are aligned below the primary ports 76. When gas burns at ports80, the resulting flame will coalesce with the flame emitted from ports76 and will provide support and stability to the larger flame, therebypreventing blowing or lifting due to high gas velocity. The distancebetween the centers of ports 76 and 80 may vary from threethirty-seconds to three-sixteenth inches.

A ledge 82 is formed in the cap 56 below the ports to help shield theflames 86. Turbulent air passing through the aperture 42 in the pan 40tends to lift the flames from the port areas. An upper ledge 84 isprovided above the ports to help shield the port area and the flames atlow "turndown.

Because of the particular configuration of the burner cap assembly 48.on the low output end of the burner operating range the burner cap 56will be heated to somewhere around 1,150 F. Experiments have shown thataluminum tends to deteriorate at these temperatures. Thus, steel is thepreferred material for the cap.

It is preferable that the burner cap 56 have a diameter of approximately2% inches and that the ports be directed through the wall 72approximately radially of the burner cap. The flames emanating from theports will tend to curl slightly upward near their ends around the upperledge 84. However, when a cooking utensil is placed on the grate 44,such as illustrated in FIG. 3, the flat bottom of the utensil 46 willtend to cause a different flow pattern for the flame 86. Because of thevelocity of the gas-air mixture ejected from the ports, the flat top capconstruction, the lack of secondary air above the cap, and the radialdirection of flow of the secondary air which flows through the aperture42 in the pan, the flames 86 will stand almost horizontally outward fromthe burner cap 56,

' substantially parallel with the flat bottom of the utensil 46.

This serves the useful purpose of preventing the flame from impinging onthe grate 44. Flame impingement on the grate tends to increase both theunburned gas and the carbon monoxide content of the resulting combustionproducts which is obviously undesirable.

In the design of the burner cap it was found that the metal walls of thecap 56 should be relatively thin, being about 0.035 to 0.050 inches inthickness, and that the cap may be formed by stamping, diecasting orother conventional mean. The thinwalled cap aspect of the invention andthe high operating temperature the cap have apparently contributed to anincreased burning efficiency of the gas-air mixture, as will begraphically illustrated subsequently. It is believed that one of thereasons for the increased efficiency is the heat which may betransmitted to the gas from the flat upper surface 70 of the cap.

As best seen in FIG. 5, when the gas-air mixture exits from the tube 50it will tend to impinge against the flat upper surface 70 of the cap andthe gas-air mixture will tend to absorb heat from the metal top 70. Thefact that turbulent flow will be caused by the change in flow directionof the gas-air mixture will tend to further increase the heat transferto the gas-air mixture. As is well known in the gas burning industry,part of the heat of combustion goes to heat the adjacent gas to ignitiontemperature. Thus, the heated gas from cap 56 will be initially hotterat the ports and absorb less heat from the adjacent flames. This uniqueheat transfer feature is clearly not the only reason for the increasedefficiency of the instant burner but it is one of the reasons.

The increased temperature on the top surface of the burner also servesanother useful purpose. With conventional burners the burner top mayattain a temperature of about 900 F. The increased burner captemperature (l,l50 F.) of the instant invention helps to incinerate andvaporize spills and droppings from the cooking utensils. The cap willthus require less cleaning and there will be a resulting lowerpercentage of the ports clogged by spilled food.

Another important feature of this invention is the openings 58intermediate the ends of the mixing tube 50. Because the conventionalair shutter has been eliminated, the inlets are less susceptible toclosure from lint, dirt and grease. However, it is obvious that whilethe air shutter has been eliminated in the preferred embodiment, an airshutter could be included without departing from the spirit or scope ofthis invention.

To show the increased efficiency of the burner cap assembly of thisinvention the testing requirements, procedures and standards of theAmerican Standard Association and the American Gas Association TestingLaboratories were used. In essence the standard requirement states thaton gas ranges the carbon monoxide produced under certain test conditionsmust not be greater than 0.08 percent air free and under the same testconditions the efficiency of the burner shall not be less than 48percent for natural gas and 50 percent for liquified petroleum gas.

In any particular burner assembly used for a gas range there is usuallya direct relationship between the amount of carbon monoxide produced andthe efficiency obtained. That is, some happy medium exists for thedistance of the burner from the bottom of the cooking utensil where boththe CO content and efficiency standards are met. Under the various testconditions required by the national standards, the latitude betweenallowable carbon monoxide produced and the required efficiency is quitesmall. Where a gain is desired in one of these two factors a sacrificemust generally be expected in the other. With conventional burners whatgenerally happens is, the burners are placed as far away from the bottomof the cooking utensil as possible to meet the minimum efficiencyrequirements. This will generally give satisfactory results on carbonmonoxide. Burner heights under these conditions usually fall within thelimits of 1% to l l/l6 inches. The term bumer heights" as used herein isintended to mean the spacing between the upper surface of the grate andthe centerline of the uppermost line of ports.

The test conditions and requirements are quite sensitive and in order topass them it is common practice on standard burners and assemblies toclosely proportion all adjustments such as the air shutter, orificesize, burner port size. secondary air distribution and burner heights.For example, on many burners the air shutter cannot be opened too far orthe flames will blow and lift from the ports. If the shutter is closedtoo far the flames are too soft and will produce carbon monoxide. Inother words, everything has to be adjusted proportionately or somefailure will result.

This is not true with the burner cap assembly of this invention.Although there are certain conditions, the operative range is muchbroader with fewer critical limitations. First, there is no air shutterat all in the preferred embodiment. Using fixed unobstructed airopenings meets all normal operating conditions. This in itself is mostunusual for this type of burner. In addition, no formed venturi sectionis used in order to aspirate sufficient primary air. And last, the heavycast burner base has been eliminated as an unnecessary element to theacceptable burning operation of the instant burner assembly.

However, the most important feature of all is the wide range of burnerheights allowable with the burner cap assembly of this invention. As canbe seen from the following data, the instant burner cap assembly may beplaced much closer to the cooking utensil (which results in much higherburner efficiencies) and while still maintaining allowable carbonmonoxide concentrations.

As can be seen from the above test data the burner height of the instantinvention is useful from five-eighths to IV: inches while theconventional burner cap is extremely limited in its efficiency and inits operating range.

The implications of this wide range of efficient uses for the instantinvention may not be obvious to the layman. However, the implicationswill come home quickly to those in the gas range manufacturing business.Because there is such a critical limitation in burner heights withconventional burners it is customary to use supplemental meansindependent of the bottom of the burner box for the purpose of mountingthe burner assembly. This is obviously an expensive operation which istotally eliminated by the instant invention.

The particular burner cap assembly of this invention may be mounteddirectly on the oven top. The expansions and contractions of the ovenwill not have the effect of raising or lowering the burner cap assemblyof this invention out of the allowable burner height range. However,with conventional burners the critical spacing between the top of thegrate and the centerline of the upper row of ports could be destroyed byflexing of the oven top when the oven is heated. Oven heat could causean expansion of the walls and top surface of the oven or perhaps abuckling of the same. This might move the burner cap vertically as muchas a quarter of an inch or in some cases perhaps even more. With theconventional burner this is obviously not permissible as may be observedfrom the test data. However, it is clearly within the allowable limitsof the burner cap assembly of this invention.

Thus, this invention allows the burner cap assembly to incorporate abracket 52 which may rest on the lower surface of the burner box or thetop of a conventional oven and the elimination of all of the prior artmounting bars and brackets which support the burner assembly on therange top itself out ofdirect contact with the bottom of the burner box.

Preferably, the volume of the chamber 74 will be approximately in therange 1.0 to 1.5 cubic inches, the total port area will be in the range0.2 to 0.3 square inches and the cross-sectional area of the mixing tubewill be approximately in the range 0.24 to 0.36 square inches. Thus, theratio of the burner head volume to the port area is approximately equalto 3.3 through 7.5 and the ratio of the burner head volume to thecross-sectional area of the mixing tube is approximately in the range2.8 to 6.3. The particular volume and cross-sectional area limitationsare critical to the gas range industry.

While the ports described and shown in the drawing are round it isobvious that other conventional shapes could be substituted; forexample, crosses, slots, elongated circles, etc.

This invention claimed is:

1. In a gas range for cooking the combination of a steel burner cap, aburner cap mounting plate and a mixing tube,

said tube having opening means for the aspiration of air into the tube,said opening means serving to provide adequate aspirated air over thefull burner operating range without means for adjusting the size of theopening means, said operating range being from a B.t.u. per hour outputof about 500 to about l2,000,

one end of said tube being adapted to receive gas injected from amanifold and the other end being connected in an opening in the mountingplate, said connection including material integral with said tube andextending radially thereof forming an annular, substantially gas tightconnection with said plate,

the cap being mounted on the plate,

said cap and plate in combination comprising a mixing chamber forcombustible gas and air delivered by said tube,

said cap including a plurality of ports directed substantially radiallyof said chamber,

ledge means above and below said ports extending radially outward forproviding flame shields at the outer extent of said ports.

2. The combination of claim 1 wherein the top of the cap issubstantially flat and continuous inwardly of its periphery.

3. The combination of claim 1 wherein said tube is of substantiallynondecreasing crosssectional area from said openings to said plate.

4. The combination of claim 1 including a grate mounted above said cap,a pan on said grate, the spacing between the top of said grate and thecenter line of the uppermost ports being in the range five-eighths to 1%inches, said combination operating within the range five-eighths to 1%inches giving off no more than an air free 0.08 percent CO and having aheat transfer efficiency to said pan of not less than 50 percent.

5. The combination of claim 1 including a bracket means adapted formounting said combination on the top surface of an oven of a gas range.

6. The combination of claim 1 wherein the ratio of the volume of themixing chamber to the cross-sectional area of the ports is ap roximatelin the range 3.3 to 7.5.

7. The com ination 0 claim 1 wherein the mixing tube [5 of substantiallyuniform cross-sectional area from end to end and the ratio of the volumeofthe mixing chamber to the cross-sectional area of the tube isapproximately in the range 2.8 to 6.3.

8. In combination a mixing tube, a bracket, a burner cap mounting plateand a burner cap,

said tube having one end shaped for connection to a gas manifold and theother end connected to the mounting plate, an opening in said tubeintermediate its ends for the aspiration of primary air, the cap andplate being joined to form a mixing chamber, the ratio of the volume ofthe mixing chamber to the cross-sectional area of the ports isapproximately in the range 3.3 to 7.5,

the mixing tube being of substantially uniform cross-sectional area fromend to end and the ratio of the volume of the mixing chamber to thecross-sectional area of the tube being approximately in the range 2.8 to6.3,

the bracket being adapted for mounting said combination above a drip panofa gas range,

one end of said tube being adapted to receive gas injected from amanifold and the other end being connected in an opening in the mountingplate, said connection including material integral with said tube andextending radially thereof forming an annular, substantially gas tightconnection with said plate,

the cap being mounted on the plate,

said cap including a flat top, a plurality of ports about its peripheryand ledge means below said ports extending substantially radiallyoutward for providing a flame shield at the outer extent ofsaid ports.

9. The combination of claim 8 including a ledge on said cap both aboveand below said ports extending radially outward to provide an annularrecess at the outer extent of said ports.

i t i i

2. The combination of claim 1 wherein the top of the cap issubstantially flat and continuous inwardly of its periphery.
 3. Thecombination of claim 1 wherein said tube is of substantiallynondecreasing cross-sectional area from said openings to said plate. 4.The combination of claim 1 including a grate mounted above said cap, apan on said grate, the spacing between the top of said grate and thecenter line of the uppermost ports being in the range five-eighths to 11/2 inches, said combination operating within the range five-eighths to1 1/2 inches giving off no more than an air free 0.08 percent CO andhaving a heat transfer efficiency to said pan of not less than 50percent.
 5. The combination of claim 1 including a bracket means adaptedfor mounting said combination on the top surface of an oven of a gasrange.
 6. The combination of claim 1 wherein the ratio of the volume ofthe mixing chamber to the cross-sectional area of the ports isapproximately in the range 3.3 to 7.5.
 7. The combination of claim 1wherein the mixing tube is of substantially uniform cross-sectional areafrom end to end and the ratio of the volume of the mixing chamber to thecross-sectional area of the tube is approximately in the range 2.8 to6.3.
 8. In combination a mixing tube, a bracket, a burner cap mountingplate and a burner cap, said tube having one end shaped for connectionto a gas manifold and the other end connected to the mounting plate, anopening in said tube intermediate its ends for the aspiration of primaryair, the cap and plate being joined to form a mixing chamber, the ratioof the volume of the mixing chamber to the cross-sectional area of theports is approximately in the range 3.3 to 7.5, the mixing tube being ofsubstantially uniform cross-sectional area from end to end and the ratioof the volume of the mixing chamber to the cross-sectional area of thetube being approximately in the range 2.8 to 6.3, the bracket beingadapted for mounting said combination above a drip pan of a gas range,one end of said tube being adapted to receive gas injected from amanifold and the other end being connected in an opening in the mountingplate, said connection including material integral with said tube andextending radially thereof forming an annular, substantially gas tightconnection with said plate, the cap being mounted on the plate, said capincluding a flat top, a plurality of ports about its periphery and ledgemeans below said ports extending substantially radially outward forproviding a flame shield at the outer extent of said ports.
 9. Thecombination of claim 8 including a ledge on said cap both above andbelow said ports extending radially outward to provide an annular recessat the outer extent of said ports.